Ethernet is the preferred protocol for many types of networks because it is flexible, decentralized, and scalable. Ethernet comprises a family of frame-based computer networking technologies for local area networks (LANs), and defines a number of wiring and signaling standards for the Physical Layer of the Open Systems Interconnection (OSI) networking model and a common addressing format and Media Access Control (MAC) at the Data Link Layer. Ethernet is flexible in that it allows variable-sized data packets to be transported across different types of mediums using various nodes each having different transmission speeds.
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fibers or electrical interfaces. Due to SONET/SDH protocol neutrality and transport-oriented features, SONET/SDH is used for transporting substantially large amounts of telephone calls and data traffic over the same fiber or wire without synchronization problems. SONET/SDH network transmission standards are based on time-division multiplexing (TDM). TDM is a technology where two or more signals or bit streams are apparently transferred simultaneously as sub-channels in one communication channel but physically take turns on the channel. This is achieved by dividing the time domain into a plurality of recurrent timeslots, e.g. of about same length, one for each sub-channel. As such, one TDM frame corresponds to one timeslot per sub-channel. In networks that use SONET/SDH and/or TDM, such as some Ethernet networks, the nodes synchronize their transmissions by exchanging a plurality of time references.